Performance Clinic - Questions and Answers - May 2014

Crankin’ Too Low

I had a professional engine builder rebuild the 426 Hemi for my Charger. It was going to be a hot street engine, with a goal of over 500 horsepower. It was by no means a budget build, and in fact, I kept getting talked into doing more and more, and ended up well into the five figures by the time I got my engine back. The engine sat around for several years before I finally got it in the car and running. The problem is, the engine is a dog. The idle quality is terrible, I get a puff of smoke every time it starts, and I would be surprised if it can run better than a 15-second quarter-mile.

Unfortunately, the shop that built the engine is now gone, so I’m working to get this fixed on my own dime. One of the tests that I did was to check the cranking compression and I only have about 120 psi per hole. I think that has to be very low for a hot street engine, and is part of the problem. Where do I go from here?

A good indicator of the engine’s ability as a pump is the cranking compression. This number provides an indication of the trapping efficiency at low rpm, a useful number in determining what is going on with the engine combination. With any new engine build the cranking compression gives a benchmark, one that you can use to determine the overall engine specifications as balanced against the static compression ratio and pressure leakage. Common theory is that the dynamic compression ratio as illustrated by the cranking compression can be manipulated by the valve timing events.

Experience tells us there is a strong correlation between the parts combination and what she cranks, and the torque output, particularly lower in the rpm range. Ideally, what we want, is to achieve high cranking compression and torque, without getting into detonation. Numerous engine building tricks will help in that direction, but to get a good indication of the execution here, the cranking compression is a good yardstick. With only around 120 psi, I’d say you are way too soft on cylinder pressure for a performance engine. The question that needs answering is why.

The key areas here are leakage, valve timing, and static compression ratio. I would check these things out in that order, starting with a leak-down test. If the valves or rings are not sealing, you are going to find it with a leak-down tester. Next, confirm the valve timing. Hopefully you know what cam was used in the build, but even if the cam is unknown, you can figure out the installed centerline and duration at .050-inch by confirming TDC and indicating off the intake lifter. Finally, you might just not have an adequate compression ratio. To fix the compression ratio you are going to have to go back into that Hemi and make some serious changes.